Move OutputAssets length check into new method (vercel/turborepo#6194)

### Description

vercel/turborepo#6191 (comment)

### Testing Instructions

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Closes WEB-1793

Author: jridgewell

crates/turbopack-core/src/chunk/chunk_group.rs

@@ -182,9 +182,5 @@ async fn references_to_output_assets(
         .copied()
         .filter(|&asset| set.insert(asset))
         .collect::<Vec<_>>();
-    Ok(if output_assets.is_empty() {
-        OutputAssets::empty()
-    } else {
-        OutputAssets::new(output_assets)
-    })
+    Ok(OutputAssets::new(output_assets))
 }

crates/turbopack-core/src/output.rs

@@ -25,7 +25,11 @@ pub struct OutputAssets(Vec<Vc<Box<dyn OutputAsset>>>);
 impl OutputAssets {
     #[turbo_tasks::function]
     pub fn new(assets: Vec<Vc<Box<dyn OutputAsset>>>) -> Vc<Self> {
-        Vc::cell(assets)
+        if assets.is_empty() {
+            OutputAssets::empty()
+        } else {
+            Vc::cell(assets)
+        }
     }
 
     #[turbo_tasks::function]